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The EDELWEISS-II experiment
Silvia SCORZAUniversité Claude Bernard- Institut de Physique nucléaire de Lyon
CEA-Saclay DAPNIA/DRECAM (FRANCE), CNRS/CRTBT Grenoble (FRANCE),CNRS/IN2P3/CSNSM Orsay (FRANCE), CNRS/IN2P3/IPN Lyon (FRANCE), CNRS/INSU/IAP Paris (FRANCE), CNRS-CEA/Laboratoire Souterrain de Modane (FRANCE), JINR Dubnia (RUSSIA), FZK/Universtat Karlsruhe (GERMANY)
Outline
Limits of EDELWEISS-I
EDELWEISS-II setup Current situation (3 detector types)
Outlook
Direct Search Principle
Detection of the energy deposited due to elastic scattering off target nuclei
• Event Rate :
< 1 ev /kg/week
• Recoil Energy :
1 – 100 keV
• Low energy threshold
◊ Large detector mass
• Low background
Radio – purity
Active/passive shielding
Deep underground sites
EDELWEISS @ Laboratoire Souterrain de Modane
Ge Heat-Ionization Detectors
Simultaneous measurements: Ionization @ few V/cm with Al
electrodes Heat @ 17 mK with Ge/NTD
sensor Different Ionization/Heat
energy ratio for nuclear and electronic recoils (dominate bkg)
Event by event background rejection
=> discrimination /n > 99.9% for Er>15keV
Edw-I limiting backgroundPRD71, 122002 (2005)
Background in the physics data taking
Leakage from the to the recoil band Neutrons: - 1 n-n coincidence observed
- Neutron shield not thick enough
Surface electron recoils:
- Bad charge collection (trapping and recombination)
- Rate compatible with 210Pb contamination:
rate “e-” 5/kg.day
E=5.3MeV Q=0.3
206Pb
210Pb 210Po
Best Sensitivity up to 2003, but…
EDW – II set-up radiopurity
dedicated HPGe detectors for systematic checks of all materials
clean room (class 100 around the cryostat, class 10000 for the full shielding)
deradonized air (from NEMO-3) neutron shielding
20 cm Pb shielding 50 cm PE and better coverage
active veto (>98% coverage)
up to 110 detectors Ge/NTD + Ge/NbSi + new ID
electrodes
expected sensitivity: EDW-I × 100 -n 10-8 pb (phase 100) = 0.002 evts/kg/day (Er>10keV)
23*320g Ge/NTD : Developed by CEA Saclay and Canberra-Eurisys Amorphous Ge and Si sublayer: better charge collection for surface evts Optimized NTD size and homogeneous working T (16-18 mK) : goal keV resolution New holder and connectors (Teflon and copper only)
All @ LSM
Ge-NTD detectors
EDW-I rate: ~ 700 evts/kg/day for E>100keVAfter the most recent improvement EDW-II rate: ~ 300 evts/kg/day
EDW-I rate on central electrode : 2.5 - 5.5 evts/kg/day (400-
1000 /m2/day)EDW-II rate
~ 3 evts/kg/day
first low bg runs in 2006:
old bg
new bg
7*400g Ge/NbSi detectors :• developed by CSNSM Orsay• NbSi thin film thermometers for active surface evt rejection
2 @ LSM
Ge/NbSi detectorsidentification of surface events with Ge/NbSi detector
data taken with 1 NbSi detector May&June 2007: 1.5kg.d (fiducial)
200
150
100
50
0
806040200
x10-3
200
150
100
50
0
806040200
x10-3
Transient
Thermal
Heat signal of thermometer NbSiA :Surface eventBulk event
Thermometer Thermometer NbSiNbSiBB
Thermometer Thermometer NbSi NbSi AA Thermometer Thermometer NbSi NbSi AA
Thermometer Thermometer NbSiNbSiBB
Time (ms)
200
150
100
50
0
806040200
x10-3
200
150
100
50
0
806040200
x10-3
Transient
Thermal
Heat signal of thermometer NbSiA :Surface eventBulk event
Thermometer Thermometer NbSiNbSiBB
Thermometer Thermometer NbSi NbSi AA Thermometer Thermometer NbSi NbSi AA
Thermometer Thermometer NbSiNbSiBB
200
150
100
50
0
806040200
x10-3
200
150
100
50
0
806040200
x10-3
Transient
Thermal
Heat signal of thermometer NbSiA :Surface eventBulk event
Thermometer Thermometer NbSiNbSiBB
Thermometer Thermometer NbSi NbSi AA Thermometer Thermometer NbSi NbSi AA
Thermometer Thermometer NbSiNbSiBB
Time (ms)Rejection works, resolutions needs tuning
ID detector
Radius (cm)
Z (
cm
)
guard ‘g’: + 1V
guard ‘h’: - 1V
‘a’ electrodes: +2V ‘b’ electrodes:
+1V
‘c’ electrodes:-2V‘d’ electrodes: -1V
Electron trajectories
holetrajectories
‘a&c’ bulk event
‘a&b’ near- surface event
‘a,b&c’ event in low-field area
200g detector, concentric electrodes
2mm gaps, 200µm width
trigger thresh. 24keV241Am source with 60keV ´s
all eventsafter cut “b” < 5keV(30% of evts. survive)
recoil energy [keV] recoil energy [keV]
lab data
preliminary!
status ID bolometer
~1.7 kg.d
summary:first detector (200g) withinterleaved electrodes in LSM performance as expected
New 400g ID´s to be installedend of March
EDELWEISS-II next steps
Goal 2x10-8 pb (1200 kg.d no event) (~1 evt/kg/y)
acquire physics data with 23 NTD build 6 400 g NbSi
Goal 2x10-10 pb (~10 evt/t/y)
build 16 NbSi (same/tuned if needed) build 24 NTD with reduced contamination and/or with ID electrodes